1. Field of the Invention
The present invention generally relates to a fault detecting system capable of detecting an accurate position on a local area network (LAN), where a fault occurs. More specifically, the present invention is directed to such a fault detecting system adapted to the 802.4 carrier band system regulated by IEEE.
2. Description of the Related Art
In FIG. 1, there is shown a typical signal transmission system known as a local area network (referred to as a "LAN").
In the signal transmission system, there are provided two data bus lines B1 and B2 for a duplexing data transmission purpose, and also a plurality of stations ST1, ST2, ---, STn ("n" being an integer) connected parallel to both the data bus lines B1 and B2.
FIG. 2 schematically illustrates an internal circuit arrangement of a first station ST1 employed in the conventional LAN shown in FIG. 1, which is similar to that of each station ST2, ---, STn.
The first station of LAN includes a higher layer "C1", a MAC layer "C2", a reception line selector "C3", a switch "SW", a combination of driver "D1" and receiver "R1" for the first bus line B1, and also another combination of driver "D2" and receiver "R2".
A transmission signal derived from the physical layer C1 employed in the first station ST1 is simultaneously transmitted via the drivers D1 and D2 to both the first and second bus lines B1 and B2. A reception signal which has been transmitted from other stations ST2, ---, STn and received via the receivers R1 and R2 from the first and second bus lines B1 and B2, is selected by way of the switch SW under the control of the reception line selector C3. Accordingly, only the signal transmitted from the normal bus line is selected and then inputted into the MAC layer C2.
In such a conventional duplexing type LAN system, when a fault, e.g., line disconnections and shortcircuits happen to occur in any one of these bus lines B1 and B2, a quick detection is required to search an accurate place where such a fault occurs.
In accordance with one conventional fault detecting apparatus, the pulse signal is transmitted from a specific position within the local area network, a time period between a transmission of the pulse signal and a reception of a reflected signal is measured so as to detect the place where the fault occurs.
Generally speaking, a signal strength of this reflected signal is very weak, because the pulse signal is reflected by the mismatching of the faulty bus line. Thus, the above-described conventional fault detecting apparatus can hardly detect such a reflected signal under such a condition that the normal transmission signal simultaneously appears on the faulty local area network. That is to say, the reflected signal is mixed with the normal transmission signal. As a result, to practically operate the conventional fault detecting apparatus, the fault detecting operation is performed either under such a condition that the operations of the overall stations ST1, ST2, ---, STn are stopped to interrupt system functions, or under such a state that LAN where the fault may occur is disconnected from all of the stations ST1, ST2, ---, STn even though the system still functions.
However, if the operations of the entire stations are stopped, the specific advantages achieved by the duplexing type LAN system are deteriorated. On the other hand, if only LAN where the fault may occur is disconnected from the overall stations, the above-described merits of the duplexing LAN system may be protected. Nevertheless, since most of stations are arranged in a discrete condition in the duplexing LAN system, there is another problem that a very long time is necessarily required to disconnect the faulty LAN.
The present invention has been made to solve the above-described problems of the conventional fault detecting apparatus, and therefore has an object to provide a fault detecting system capable of detecting an accurate place in a duplexing type LAN where a fault may occurs, under a condition that the overall system is still operable. In other words, the present invention has an aim to provide a fault detecting system suitable for the IEEE's carrier band type LAN system.